Subnetting

Efficiency
Examples
Configurator

Exercise 1: Class B, 500 subnets

• Class B addresses 119.48.0.0 (in CIDR-speak, 119.48./16)
• 500 subnets are required

• How many bits are required to create 500 subnets?
• The number of subnets available is 2ⁿ - 2, where n is the number of 'borrowed' bits. The 2 subtracted is to allow for the network address (which cannot be a subnet) and the network broadcast (which cannot be a subnet). For more information see Subnetting Efficiency
• 2⁸ - 2 = 253. Not enough.
• 2⁹ - 2 = 510. Bingo!
• The first 16 bits are assigned, the next 9 are subnetwork addresses. Any differences in these first 25 bits means the addresses are on different networks (and must be routed by the router).
• The subnet mask is 25 1's followed by 7 0's:
  11111111 . 11111111 . 11111111 . 10000000
  Subnet mask is 255.255.255.128
• The number of host addresses available on each subnet is 2^m - 3, where m is the number of host bits (n + m + assigned bits = 32). The 3 subtracted is to allow for the network address (which cannot be a host), the broadcast (which cannot be a host) and the gateway (which cannot be a host). For more information see Subnetting Efficiency
  2⁷ - 3 = 125

• 300th usable 9-bit subnet is 10010110 0.
• The subnet 300 address is
  119 . 48 . [ 1 0 0 1 0 1 1 0  .  0 ] { 0 0 0 0 0 0 0 }
• Gateway will be the first usable address on subnet 300:
  119 . 48 . [ 1 0 0 1 0 1 1 0  .  0 ] { 0 0 0 0 0 0 1 }
  Gateway address is 119.48.150.1
• Subnet mask is 255.255.255.128 (from above)
• Host addresses start from the second usable address (the first is the router)
  119 . 48 . [ 1 0 0 1 0 1 1 0  .  0 ] { 0 0 0 0 0 0 1 }
  First host's address is 119.48.150.2

Exercise 2: 20 assigned bits, 100 subnets

• CIDR addresses 155.70.144/20
• 100 subnets are required

• How many bits are required to create 100 subnets?
• The number of subnets available is 2ⁿ - 2, where n is the number of 'borrowed' bits. The 2 subtracted is to allow for the network address (which cannot be a subnet) and the network broadcast (which cannot be a subnet). For more information see Subnetting Efficiency
• 2⁶ - 2 = 62. Not enough.
• 2⁷ - 2 = 126. Bingo!
• The first 20 bits are assigned, the next 7 are subnetwork addresses. Any differences in these first 27 bits means the addresses are on different networks (and must be routed by the router).
• The subnet mask is 27 1's followed by 5 0's:
  11111111 . 11111111 . 11111111 . 11100000
  Subnet mask is 255.255.255.224
• The number of host addresses available on each subnet is 2^m - 3, where m is the number of host bits (n + m + assigned bits = 32). The 3 subtracted is to allow for the network address (which cannot be a host), the broadcast (which cannot be a host) and the gateway (which cannot be a host). For more information see Subnetting Efficiency
  2⁵ - 3 = 29

• 3rd usable 7-bit subnet is 0000 011.
• The subnet 3 address is
  155 . 70 . 1 0 0 1 [ 0 0 0 0  .  0 1 1 ] { 0 0 0 0 0 }
• Gateway will be the first usable address on subnet 3:
  155 . 70 . 1 0 0 1 [ 0 0 0 0  .  0 1 1 ] { 0 0 0 0 1 }
  Gateway address is 155.70.144.97
• Subnet mask is 255.255.255.224 (from above)
• Host addresses start from the second usable address (the first is the router)
  155 . 70 . 1 0 0 1 [ 0 0 0 0  .  0 1 1 ] { 0 0 0 1 0 }
  First host's address is 155.70.144.98

© Copyright 2000 Guy T. Schafer, Cisco-Certified Network Associate
Last updated January, 2000